In production paint operations, painters change the color of the sprayed paint by disconnecting their spray guns from one paint supply line, paint drop, and connecting it to another paint drop of a different color. The old paint color is purged by discharging a small amount of the new color through the spray gun.
In contrast to manual painting, automated painting machines require the addition of automatic color changers. These devices use valves, manifolds, and occasionally long tubes to connect different color paints to the spray gun, purge the lines, valves, manifolds, and the spray gun with solvent, and refill with a new color paint. The long tubes cause considerable waste of valuable paint material and the use of much solvent to purge and clean the lines during color changes. Environmental concerns have been raised about the disposal of such materials and a more economical way for changing paint color has been eagerly sought.
One option for addressing environmental concerns has been the charging of the painting material with a high voltage electrostatic charge, exceeding 100 KV, which helps improve its coating efficiency and the economics of painting. Solvent based paints have been successfully sprayed with electrostatic charge by virtue of their natural insulating properties.
Another desirable option for addressing the environmental concerns has been the use of water based paints which pose much less threat to the environment than solvent based paints. Unfortunately, such paints are conductive of electricity and can be sprayed electrostatically only from insulated canisters, disconnected from the paint supply lines. The filling, purging, cleaning, and refilling of such canisters with different paint colors has been a challenge looking eagerly for economic solutions, particularly for high volume production operations requiring frequent color changes.
U.S. Pat. No. 4,313,475 to Wiggens addresses these challenges by means of a system of supply lines, color changers, valving arrangements, and air entrapping containers to fill the containers from the supply line, when the electrostatic charge is turned off. Valves are actuated to cause a discontinuity in the supply line, termed voltage block, that isolates it from the charged paint in the canister. Air entrapped in the container and pressurized by the filling of the canister causes the paint to flow from the canister to the spray equipment while the paint is electrostatically charged.
U.S. Pat. No. 4,771,729 to Planert et al; U.S. Pat. No. 4,932,589 to Diana; U.S. Pat. No. 4,879,137 to Behr et al; and U.S. Pat. No. 4,921,169 to Tilly all disclose various methods and apparatus for electrostatically coating a conductive material utilizing an electrical isolation mechanism, including a reservoir into which a metered volume of paint to be sprayed is introduced.
With the advent of robots, production spray painting became more efficient and attained higher quality. Color changing for robots followed the lead of automatic spray machines by using automatic color changers. However, the mobility of the robot also caused the spray lines to become longer and require more solvents to purge and clean. Electrostatics also cause damage to the long flexible supply lines as they get in contact with grounded surfaces, and eventually fail. To continue the use of electrostatics, especially with water based paints, a better method is sought for paint delivery, color changing, and electrostatic isolation, for robotic paint spray equipment.
The current approach is to connect a spray applicator by means of fluid lines to an outlet port of a paint color changer. The color changers is piped to a series of paint outlets (color drops). The color changer controls the flow of paint, air and cleaning solvents to the applicator by means of a valving arrangement. Valves are selectively opened or closed to pass the appropriate paint color to the applicator, to purge the fluid lines with air or solvents, and to clean the lines between.
Most methods of delivering paint by robots to a workpiece keep the robot connected to the supply lines by means of hoses. The length of the hoses causes paint to be wasted during color change, waste of robot time when idled for purging and color change, and restricts robot mobility.
For electrostatic painting with conductive paints, such as water based paints, the charging of the paint causes the high voltage charge to be communicated to the rest of the paint supply system through the paint lines, hence impeding the use of such paints despite their environmental advantages.
Most current methods for painting with robots require the use of automatic color changers with a multiplicity of valves and control solenoids to connect the appropriate color to the spray gun of the robot and to provide sequenced operations for color changing and line cleaning. This is not always a necessity and adds expense and complexity to the painting process.
In contrast with the above, manual painting requires no color changers as the person has the ability to connect his spray gun to the source of the desired color directly and disconnect it when he is finished using that particular color. Cleaning is also limited to the spray nozzle since the paint delivery lines need not be cleaned as they do not pass different color paint, hence manual painting uses little cleaning solvents. Furthermore, less time is spent to switch from one color to another, without paint line cleaning, allowing more time for productive painting. However, manual painting is known to be deficient in quality.
U.S. Pat. No. 4,785,760 to Tholome discloses a sprayer installation including a robot, a sprayer carried by the robot and a storage tank for a fluid, such as paint to be sprayed, also carried by the robot.
The mobility of the robot simplifies its operation and eliminates the need for automatic color change equipment. Mobility of the robot is also used to eliminate the need for special insulating provisions to isolate the spray equipment from the paint supply lines. The robot mimics manual painting operations rather than automatic machine operations, hence, requiring no automatic color changers, and no special voltage blocks for periods of electrostatic spraying.
U.S. Pat. No. 3,674,207 to Carbonetti, Jr. et al; U.S. Pat. No. 4,944,459 to Watanabe et al; and U.S. Pat. No. 5,029,755 to Schmidt et al all disclose the use of a robot or manipulator which is utilized with other apparatus to change paint color.